The present invention relates to device, system and method for stereotactic medical procedures. More specifically, it provides for repeated accurate positioning (fixation) of a patient or part of a patient for carrying out medical procedures which are done at different times.
Various medical procedures involve repeated treatments at different times. For example, application of radiation is sometimes used for treating brain tumors or other conditions. Although a single application of radiation may sometimes be used, under many circumstances there are sound medical reasons to use repeated application of radiation at different times.
The treatment of a radiation therapy patient can be broken down into four stages. These are (1) diagnostic evaluation, (2) treatment planning, (3) simulation and (4) treatment. Our repeat fixation device is applicable to the latter three phases of the treatment process. In the first stage of diagnostic evaluation the physician decides which tissues are at risk of disease and should be targeted. The patient may undergo many diagnostic tests including angiography, computerized tomography (CT) and magnetic resonance (MR) imaging. After the physician is satisfied that they have identified the tissues at risk, the patient then undergoes a process known as treatment simulation. This process involves obtaining a set of images such as plane films, digital images, CT, MRI, and ultrasound images. These radiographs allow the physician to select a specific path for each radiation beam which only includes the tissues at risk and excludes normal tissues. Because the tissues the physician has targeted are often radiographically transparent the physician routinely relies upon radiographic landmarks to infer the proper beam alignment. These same landmarks are subsequently imaged on similar radiographs taken with the therapeutic x-ray beam prior to administering the radiation treatment. These pretreatment radiographs, which are known as therapy portal films, allow the physician to judge the appropriate alignment of the treatment beam and the patients anatomy. The frequency at which these portal films are repeated is dependent upon the complexity of the patient setup and the proximity of the beam to critical structures (such as a patient's optic nerve).
A routine course of radiation therapy may span anywhere from 10 to 64 fractions over a period of two to six weeks. The number of treatments dependent upon the specifics of the particular disease. For each fraction the patient must be repositioned at the teletherapy unit and aligned relative to the radiation beam.
There exists a clinical situation in which the target tissues cannot be adequately localized by their proximity to radiographically opaque structures as required by the above simulation procedure. Arteriovenous malformations, acoustic neurinomas and other small intracranial targets are examples of such clinical entities. To enable the identification, and subsequent treatment of such targets, a new and very powerful technique known as radiotherapy has been developed. (Radiosurgery is usually considered to be a single fraction radiotherapy treatment, meaning a single treatment, although it may also be more broadly interpreted. Multiple radiotherapy treatments are often called high precision radiotherapy or fractioned stereotactic radiotherapy.) This technique allows small intracranial targets to be identified and treated to a very high degree of precision.
The radiosurgical technique uses stereotactic principles for targeting, localization and treatment. The procedure begins with a stereotactic reference system being fixed to the patient's skull. This reference system remains fixed relative to all intracranial points throughout the entire radiosurgical procedure. All diagnostic exams, such as angiography, CT and MR scanning include a set of fiducial markers which allow all points within the image to be localized relative to the stereotactic reference frame.
Once the target tissues have been identified the path of radiation beams can be mathematically computed. The computer algorithms, which support this procedure, allow the clinician to evaluate the amount of dose which would be deposited within the patient if the simulated beams were actually x-ray beams were applied along the proposed paths. In an attempt to arrive at a treatment plan which adequately confines the radiation dose to the target tissues while limiting the dose to all normal tissues the beams of radiation are modified, eliminated or new beams added to the plan. Once a plan with an acceptable dose distribution has been arrived at the information on beam trajectory is transferred to the radiotherapy treatment unit. A single fraction of radiation is then given to the patient and the stereotactic frame is removed. The entire length of the procedure, from frame application through treatment, usually spans 6 to 8 hours.
The present inventors' prior U.S. patents listed below, assigned to the assignee of the present application and hereby incorporated by reference disclose techniques for providing stereotactic radiosurgery with a high degree of precision:
______________________________________ U.S. Pat. No. Issue Date Title ______________________________________ 5,027,818 July 2, 1991 DOSIMETRIC TECHNIQUE FOR STEREOTACTIC RADIOSURGERY 5,189,687 February 23, 1993 APPARATUS FOR STEREOTACTIC RADIOSURGERY ______________________________________
The techniques of the inventors' above patents allow the patient to be precisely positioned relative to radiation beams of stereotactic radiosurgery to within 0.2 mm plus or minus 0.1 mm. Although this works very well for single fraction therapy, there exist clinical settings where fractionating the total dose, i.e. dividing the dose into many small fractions, would yield additional therapeutic advantage. In the radiotherapy procedure, once the reference frame has been removed from the patient the relationship between intracranial target points and the reference system is lost. Because the above procedure would require the reference frame to remain fixed to the patient's skull through the entire course of treatment, which may last several weeks, this approach is considered inappropriate for fractionated therapy. Alternately, each fractional treatment would require a laborious and time-consuming procedure to redetermine patient position for second and subsequent treatments.
There exist several different techniques for non-invasive repeat fixation. These methods can be broken down into three basic categories. These are bite plate systems, contour realignment systems and mask systems. All of these systems have design flaws which can lead to unacceptable, and undetectable, positional errors.
The mask techniques have been used in radiation therapy for over three decades. In these system a custom mask, which snugly fits either the face or the entire head, is fabricated. For high precision radiotherapy the mask is then attached to a stereotactic reference frame, similar to the frame used for any stereotactic procedure. Prior to each diagnostic exam the patient is placed into the mask/frame system and normal stereotactic fiducial systems are used for image registration.
Mask immobilization and repositioning systems have been used extensively in radiation therapy. From multiple reports in the literature mask systems appear to have a repeat fixation tolerance no better than 3 to 5 mm. It is our opinion that this level of accuracy is unacceptable for fractionated radiotherapy.
Bite plate systems have also been used in radiotherapy for several decades. This technique requires the fabrication of a customized bite plate. The plate fits snugly onto the patient's teeth. As with the mask/frame systems, the bite plate is fixed to a stereotactic reference frame which then accepts the routine set of fiducial markers for both plane film radiography, CT and MR scanning. The primary disadvantage of this system is that the bite plate is used for both localization and patient fixation. The bite plate not only provides the reference for stereotactic localization, but it also is the mechanism which is used to move the patient into position. Moving the patient by use of the bite plate produces torque on the bite plate-teeth interface. An analysis of this approach reveals that very small movements in the bite plate position, relative to the patient's teeth, can result in large translations and rotations of the intracranial targets. Since no method of alignment verification has ever been developed, these errors go undetected.
An alternate system for patient positioning uses the patient's own anatomical contours as the stereotactic reference system. In this approach a CT or MR scan is taken and a three dimensional reconstruction of the patient's surface is obtained. These contours act as the reference system for stereotactic localization.
The usual diagnostic exams are carried out and the treatment is then planned using the same stereotactic principles used in routine radiotherapy. The target is identified and the patient's surface contour coordinates are measured relative to the isocenter. The patient is placed at the teletherapy treatment unit and the surface contours are again obtained through the use of surface digitization. A set of algorithms then calculate the translations as well rotations required to reposition the patient's target over the teletherapy units isocenter. The accuracy of such systems under clinical test conditions have been shown to be approximately two to three mm.
When performing fractionated radiotherapy, accuracy in applying the radiation is very important. Some tumors or other conditions require that the radiation be concentrated in relatively small volumes. Misalignment of the radiation beam may cause an insufficient amount of radiation to be applied to the tumor or other target. Further, such misalignment may increase the likelihood and/or degree of damage to healthy tissue adjacent the tumor or other target.
Fractionated radiotherapy may be imprecise if the tumor or other target cannot be localized with a sufficient degree of accuracy. However, this need for proper localization is the same need which one has when performing single dose radiotherapy and this need is addressed by the present inventors' incorporated by reference patents. The additional factor in fractionated radiotherapy is the need to easily and accurately repeat a position of the patient. If the position of the patient was accurate relative to the first treatment, the repositioning should normally cause the patient to assume the exact same position (relative to the treatment mechanism) for the second and subsequent treatments. However, if the second or other subsequent treatment is performed with the patient only slightly moved from the first treatment position, this will introduce inaccuracies. The repeat fixation techniques discussed above have the indicated disadvantages.
More generally, the need for repeat fixation of a patient or portion of a patient exists outside of radiotherapy. In the general case, one wishes to perform a first medical procedure on a patient with a precise localization of portions of the patient, and, at some later time, perform a second medical procedure on the patient with a precise localization of portions of the patient. One can repeat laborious and time-consuming localization steps for the second medical procedure, but this increases medical costs and complexity. As used herein, a medical procedure is a procedure for diagnostic and/or remedial purposes.